Among
Old English ongemang (from on ‘in’ + gemang ‘assemblage, mingling’). The -st of amongst represents -s (adverbial genitive) + -t probably by association with superlatives (as in against ).
wiktionary
From Middle English among, amang, amonge, amange, from Old English amang, onġemang, equivalent to a- + mong(“crowd; group; throng”). Compare dialectal German mang, Saterland Frisian monk, monken(“among”).
etymonline
among (prep.)
"in, in the midst of," early 12c., from Old English onmang, in late Old English sometimes amang, a contraction of ongemang "among, during," from phrase on gemang, literally "in the crowd or company (of)," from on (see a- (1)) + gemengan "to mingle," from Proto-Germanic *mangjan "to knead together," which is perhaps from a nasalized form of PIE root *mag- "to knead, fashion, fit." The collective prefix ge- was dropped 12c. leaving onmong, amang, among. Compare Old Saxon angimang "among, amid;" Old Frisian mong "among."
